1. Field of the Invention
This invention relates to a printing system, and more particularly to an apparatus for transporting sheets from a transferring station to a fixing station in a transfer type printing or copying machine.
2. Description of the Related Art
Generally speaking, in a transfer type printing system, an apparatus for transporting sheets between transferring and fixing stations is usually one of the types, transporting roller, or transporting belt, or a type of using both transporting belt and suction means, in which a sheet transporting speed is set to be constant in the transferring and fixing stations, as well as in the means for transporting the sheet therebetween so as not to give any damages to the unfixed powder images transferred on the printing sheets at the transferring station.
On the other hand, printing speed-up in a printing system has recently been required so that raising the sheet transporting speed in both the transferring and fixing stations will be necessary.
In the fixing station, however, there are usually provided fixing rollers comprising a pair of rollers such as a heat roller and a pressure roller to preserve desired heat efficiency and safety. Therefore, if transporting speed is too high at the fixing rollers, the fixing efficiency will be reduced. In order to solve the reduction of the fixing efficiency, a fixing temperature may be raised, or the pressure exerted between the fixing rollers may be raised. However, if the fixing temperature is too high, much more power will be consumed in the whole machine. In addition, there will be a limit in raising temperature for the heat endurance of the fixing rollers. On the other hand, if the roller pressure is raised, the printing sheets will be subjected to wrinkles or waves when they pass through the fixing rollers.
In order to solve the above-mentioned problems, U.S. Pat. No. 3,794,417 discloses a copy machine in which, when a sheet is passing through a transferring station, the speed of the transporting means is set to be the same as the sheet transporting speed in the transferring station, and on the other hand, after the rear edge of the sheet passes through the transferring station, the speed of the sheet transporting means is changed to be the same as the sheet transporting speed in the fixing station. Such a transporting speed control is effected by an eccentric pulley or cam rotating synchronizingly with a photo-sensitive drum, but not synchronizingly with the actual sheet movement. Therefore, if there was a delay in the time at which the rear edge of sheet should have been passed through the transferring station, the sheet would be subject to deflection or deformation between the transferring and fixing stations, which might occur damages on the powder images on the sheet. In addition, in order to proceed or use different sizes of sheets, several kinds of eccentric pullys or cams are necessary, which will make the machine more complicated.